Muhuo Yu

The Effect of Crystallization on the Solid State Polycondensation of Poly(L‐lactic Acid)

Melt solid polycondensation is an approach to increase the molecular weight of poly (L‐lactic acid) (PLLA). For this report, the effect of crystallization time of PLLA prepolymer on the molecular weight of the biomaterial was studied. In this process, PLLA prepolymer with a molecular weight of 18,000 was first prepared by the ordinary melt‐polycondensation process. The prepolymer was crystallized at 105°C for various times, and then heated at 135°C for 15–50 h for further solid state polycondensation (SSP). The differential scanning calorimetry (DSC) and viscosity measurements were used to characterize the crystalline properties and molecular weight of the resulting PLLA polymers, respectively. The results showed that the molecular weight of PLLA reached a maximum value under the condition of a crystallization time of 30 min and SSP of 35 h.

Simultaneously Enhancing Mechanical Properties and Melt Flow Ability of Polyamide 6 by Blending with a Hyperbranched Polymer

A series of polyamide 6/hyperbranched polymers (PA6/HBP) blends with different HBP contents was prepared by melt processing using a twin-screw extruder. The HBP was synthesized on the basis of pentaerythritol and dimethyl terephthalate according to a one-step method. The melt flow behavior, crystallization behavior, morphology, and mechanical properties of the PA6/HBP blends were investigated. The results showed that the melt flow index of the blends was greatly improved by a small amount of HBP. The yield strength, tensile modulus, Izod impact strength, and flexural strength of samples were simultaneously enhanced from 54.6 MPa, 0.5 GPa, 3.8 kJ/m2, 56.9 MPa for pure PA6 to 61.1 MPa, 0.7 GPa, 5.3 kJ/m2, 67.1 MPa for PA6 blends with 2.0 wt% HBP, respectively. The PA6/HBP blends showed the higher content of α-form crystal and a higher degree of crystallinity than those of pure PA6.

Precursor Based on Polyborylborazine for BN Fiber: Preparation and Rheology Properties

A polyborylborazine precursor for hexagonal boron nitride (h-BN) was obtained by reaction of boron trichloride with methylamine and its structure was characterized by 11BNMR, 13CNMR, 1HNMR, and FTIR. The results show that the molecular precursor consists of borazine rings connected via a cross-linked network. The results of shear rheological tests indicated that the polymer is capable of being melt spun at moderate temperature, which implies that the structure of the molecular chains of the precursor polymer is branched. The precursor polymer was spun into a continuous polymer fiber in the melt state and then subsequently heat-treated under NH3 up to 1000°C for conversion into BN fibers. Its surface morphology was observed by scanning electron microscopy (SEM); the fiber was free of defects and cavities.

Debinding and Sintering of an Injection-Moulded Hypereutectic Al–Si Alloy

Hypereutectic Al–Si (20 wt.%) alloy parts were fabricated by employing a powder injection moulding (PIM) technique with a developed multi-component binder system composed of high-density polyethylene (35 wt.%), carnauba wax (62 wt.%) and stearic acid (3 wt.%). The feedstocks contained 83 wt.% metal powders. The debinding process was carried out by a combination of solvent extraction and thermal decomposition. The effects of solvent debinding variables such as kind of solvents, debinding temperatures and time, and the bulk surface area to volume ratios on the debinding process were investigated. Thermal debinding and the subsequent sintering process were carried out in a heating sequence under a nitrogen atmosphere. The influences of sintering temperature and sintering time on the mechanical properties and structure were considered. Under the optimal sintering condition, sintering at 550 °C for 3 h, the final sintering parts were free of distortion and exhibited good mechanical properties. Relative sintered density, Brinell hardness, and tensile strength were ~95.5%, 58 HBW and ~154, respectively.

Microstructural and Mechanical Characterization of SiBNC Coated Carbon Fibers Prepared via PIP Method

The preparation of SiBNC ceramic oxidation-resistant coating on carbon fiber via precursor-solution infiltration and pyrolysis (PIP) method was presented and discussed in detail. Moreover, the microstructures and the components of the coating were characterized. The experiment results showed that the 0.5% polyborosilazane (PBSZ) solution in the coating preparation gave fewer defects, in contrast with the 1% and the 5% PBSZ solutions. The strength of the carbon fiber coated with SiBNC-0.5wt%(3.08GPa) was decreased by 12%. Compared with uncoated carbon fiber, the starting oxidation temperature and the reaction activation energy of the coated carbon fibers (SiBNC-0.5wt%) have increased by about 300 °C and 104%, respectively. The oxidation resistance of the SiBNC coated carbon fiber was improved dramatically.

Dissolution behaviour of polyethersulfone in diglycidyl ether of bisphenol-A epoxy resins

Polyethersulfone (PES) is considered as an important and useful toughener for epoxy resins (EPs). Generally, PES was first dissolved in EP oligomer to form a homogeneous solution, which considerably affected the properties of cured EPs. Turbidity measurement was carried out to characterize the dissolution behaviour. Different PES/EP systems exhibited the same dissolution behaviour, albeit different swelling rate (R S) and dissolution rate (R D). Compared to molecular weight, the particle size and hydroxyl content of PES exerted considerable effects on dissolution. The smaller epoxy equivalent these diglycidyl ether of bisphenol-A (DGEBA) oligomers possessed, the better dissolubility they had; the more small molecules they contained, the more apparent the swelling observed. Moisture prevented the diffusion of the DGEBA oligomer molecules into PES. With increasing temperature, R S and R D first rapidly increased and then slowly increased, and the optimal dissolution temperature was 120°C. At the final steady, the turbidity increased linearly with increasing PES concentration; in addition, these values can be used as reference for homogeneous solution. Only one glass transition temperature (T g) was observed for PES/EP systems, indicative of the good compatibility between PES and EP.

Development of Hypereutectic AlSi Alloy Powder Injection Molding Feedstocks by Rheological Analysis

The comprehensive properties of a feedstock have a critical influence on the powder injection molding process. Proper feedstock with homogeneous structure, favorable flow characteristic, and moldability is the prerequisite for obtaining a final part with excellent comprehensive properties. The objective of the present work was to develop an optimal feedstock for fabrication of hypereutectic AlSi (20u2009wt.%) alloy parts by the powder injection molding technique. For this purpose, micron-sized hypereutectic AlSi (20u2009wt.%) alloy powder was mixed with different amounts of a binder which consisted of 35u2009wt.% high-density polyethylene, 62u2009wt.% carnauba wax, and 3u2009wt.% stearic acid. The binder contents of the feedstocks were in the range from 13u2009wt.% to 21u2009wt.%. The influences of binder content, shear rate, and temperature on the rheological behaviors of feedstocks have been investigated via a capillary rheometer. The feedstock with 21u2009wt.% binder exhibited a variable flow behavior and was culled. The rest of the feedstocks showed a pseudoplastic behavior. Comprehensive analysis of rheological parameters such as the flow behavior index, yield stress, flow activation energy, and the general moldability index, the feedstock with 17u2009wt.% binder exhibited the best rheological performance and favorable moldability. The molded part with 17u2009wt.% binder had constant density, good shape retention, and stiffness as well as homogeneous distribution of the powder and binder. After solvent debinding, the debound item showed a homogeneous porous structure which is suitable for the subsequent thermal debinding and sintering processes.